1. Field of the Invention
The present invention relates generally to stress measurement devices for bolt type fasteners. More specifically, the invention discloses an ultrasonic transducer assembly incorporated into a printed circuit board and for determining, specifically during installation or diagnostic testing, tensional holding forces experienced by the bolt during such tightening action. The transducer assembly functions by securing to a head of the fastener and emitting high frequency shock waves (longitudinal and/or transverse waves) through the fastener""s axial length. An established flight time, i.e., the time necessary for the ultrasonically created shock wave to travel in forward and return directions along the bolts length, is correlated to a particular level of tension in the bolt. The stress measurement device further provides aspects of reduced expense and disposability not found in the prior art.
2. Description of the Prior Art
Referring to FIGS. 1 and 2 of the illustrations, an example of a prior art and transducer mounted tension measurement device is illustrated at 2. The transducer measurement device 2 includes a substantially steel cylindrical casing 4, typically constructed of a steel and may be surrounded by a magnetic material or other non-magnetic material, and which is secured by a couplant (such as water/glycerine mixture, epoxy, or other suitable type of glue) atop a head 6 of a bolt-type fastener 8. Even in instances where a magnetic casing is employed, it has been found necessary in the prior art to apply a suitable adhesive due to the tendency of the magnetically adhered device to laterally move or xe2x80x9cwalkxe2x80x9d upon the bolt head surface and if not properly affixed in place.
Additional components of the prior art device 2, and referring specifically again to FIG. 2, include a bottom facing wear plate 10, a piezo-ceramic element 12, internal backing material 14, miniature wire harness 16 and connector 18 for communicating the wire harness to the device 2. Not shown is a PC device operatively communicated by an input plug located at a remote end (also not shown) of the wire harness 16.
The bolt 8 typically is employed in a tightening operation to adhere together first 20 and second 22 surfaces and usually includes a first washer 24 (compressed by the bolt head 6 and located on one side of the first surface 22) and a tightening nut 26 (located on an opposite side of the second surface 24) and compressing therebetween a second washer 28. In use, the piezo-ceramic element 12 is energized by a signal from the remote PC and proceeds to emit an ultrasonic wave pattern which travels, in a first direction 30 (see again FIG. 2), the substantial axial length of the bolt fastener 8 and, in a second direction 32, a return length 32.
The connector and associated wire harness of the device 2 transmits a signal represent the combined travel (or xe2x80x9cflightxe2x80x9d) time of the ultrasonic wave along directional arrows 30 and 32 to a software/hardware program (also not shown) incorporated into the PC. The signals correspond to a time shift in the return direction 32. The program incorporates suitable subroutines for correlating the input signal to an output representative of a determined amount of tension or force (exercised upon the bolt in opposite directions 34 and 36) during tightening of the bolt 6. In this fashion, an amount of tension in the bolt can be measured (or if already determined avoided) during both the manufactured assembly or diagnostic applications.
Upon conclusion of the bolt fastener 8 diagnosis, the transducer device 2 is separated from the head 6 by physically overcoming the magnetic attraction between the device 2 and the bolt 8 or, as in many situations where a suitable epoxy is employed, the device 2 is forcibly disengaged from the bolt head 6. The prior art device 2, while providing an effective mechanism for measuring bolt tension, suffers from the shortcoming of its significant cost of construction, which necessitates its repeated reuse with different fasteners. This in turn renders problematic the practical requirement that suitable epoxy and other types of couplant be employed to ensure that the device 2 is securably attached to the bolt hold.
Additional types of ultrasonic load indicating assemblies are known in the art, examples of which are illustrated in U.S. Pat. Nos. 4,899,591; 4,846,001 and 5,131,276, all issued to Kibblewhite, and which teach a thin piezoelectric sensor and method of assembly consisting of a piezoelectric film sandwiched between two thin electrodes. The piezoelectric film is permanently mechanically and acoustically coupled to the upper surface of a member and is used to determine the length, tensile load, stress or other tensile load dependent characteristic of the member by ultrasonic techniques.
U.S. Pat. No. 5,437,525, issued to Bras, teaches an assembly component having a force measuring sensor which includes first and second faces and an axis of symmetry with a ceramic support layer having a mounting side carrying a piezoresistive layer having a resistance that varies in response to its deformation. The force sensor farther includes a central metallic layer and an annular metallic layer joined by the piezoresistive layer or at least two separate sensors having angularly disposed longitudinal axes. A machine for tightening the assembly component includes a motor driven socket for torquing the component in accordance with a sensed tightening tension.
Finally, U.S. Pat. No. 2,600,029, issued to Stone, discloses a stress indicating bolt or stud with a longitudinal bore, within which extend calibrated fine resistance wires. Measurement of bolt stress is achieved by passing an electric current through a series circuit formed by the fine resistance wires and the variations in resistance which are determined are correlated to actual stress experienced by the bolt.
The present invention is an ultrasonic transducer assembly incorporated into a printed circuit board and which is an improvement over prior art devices for measuring the tightening tension experienced by bolt-type fasteners. More particularly, the present invention provides a unique, reduced cost and, at the user""s option, disposable tension measurement device. Specifically, the transducer assembly can be less expensively produced employed mass insertion and soldering techniques, more quickly inspected, and more easily applied to a desired bolt fastener. The relative low cost of the transducer assembly, and in particular relative to prior art transducer devices, permits its discard after completion of bolt diagnosis.
The assembly includes a printed circuit board (PCB), preferably manufactured in multiple fashion within in a template layout. Each PCB incorporates at least one circuit configuration for communicating first (top) and second (bottom) faces of the PCB. In a first preferred embodiment, a connector secures to a first location of the first face and bolt identification component, either resistor or ID chip circuits, connects to a second location of the first face. The connector receives an input end of a wire harness extending from a personal computer (PC) or other suitable processing device and, depending upon whether a resistor or multi-bit ID chip employed in the circuit arrangement for electrically communicating with the PC and determining the identity of the bolt to be analyzed, either a two-pin or three-pin connector is used.
By manufacturing the PCB components within the template layout, a multi-port input can be configured at an appropriate location of the template (and by which each input communicates with a specified PCB). A suitable and multi-pin connector extends from the PC and engages the multi-port input. The PC incorporates suitable software for sequentially analyzing each of the PCB units and this is typically accomplished by first submerging the template in a volume of fluid and then by systematically energizing the piezo-ceramic associated with each printed circuit board. In this fashion, the status (good/bad) of each PCB unit can be determined. Break-off points are incorporated between each printed circuit board and the template to enable the PCB to be quickly detached for either disposal (bad) or application (good) in ultrasonic measurement.
Prior to application upon the fastener head, a conforming material, typically a volume of an injected plasticized/resinous material, is applied to the piezo-ceramic and at least substantially the second side of the assembled printed circuit board. In a further application, a substantially annular shape member is employed for insertably engaging the assembled PCB unit and a fluidic couplant may additionally be injected into the annular shaped member prior to engagement of the PCB unit.
The assembled transducer assembly is then secured, with the use of a suitable fluid adherent atop the head of the bolt fastener to be ultrasonically measured. The adherent can also be selected from both conductive and non-conductive materials and examples of suitable adherents include epoxies or glues and it is further contemplated that an adherent exhibiting a specified and lesser holding force (such as provided by weaker adhesives) can be selected depending upon whether the user desires to forcibly disengage the PCB unit from the fastener head for subsequent reuse.